1. Field of the Invention
The present invention relates to a light-emitting device and to a method of making the same.
2. Related Technology
In the last decade, much effort has been devoted to the improvement of the emission efficiency of light-emitting devices (LEDs) either by developing highly efficient materials or efficient device structures.
FIG. 1 shows cross-section of a typical LED. The device has an anode 2, a cathode 5 and a light emissive layer 4 located between the anode and the cathode. The anode may be, for example, a layer of transparent indium tin oxide. The cathode may be, for example, LiAl. Holes and electrons that are injected into the device recombine radiatively in the light emissive layer. A further feature of the device is the optional hole transport layer 3. The hole transport layer may be a layer of polyethylene dioxythiophene (PEDOT) for example. This provides an energy level which helps the holes injected from the anode to reach the light emissive layer.
Known LED structures also may have an electron transport layer situated between the cathode 5 and a light emissive layer 4. This provides an energy level which helps the electrons ejected from the cathode to reach the light emissive layer.
In an LED, the electrons and holes that are injected from the opposite electrodes are combined to form two types of exitons; spin-symmetric triplets and spin-anti-symmetric singlets. Radiative decay from the singlet (fluorescence) is fast, but from the triplet (phosphorescence) is formally forbidden by the requirement of the spin conservation.
In the past few years, many have studied the incorporation of phosphorescent materials into the light emissive layer by blending. Often, the phosphorescent material is a metal complex, however it is not so limited. Further, metal complexes also sometimes are fluorescent.
A metal complex will comprise a metal ion surrounded by ligands. A ligand in a metal complex can have several roles. The ligand can be an “emissive” ligand which accepts electrons from the metal and then emits light. Alternatively, the ligand may be present simply in order to influence the energy levels of the metal. For example, where emission is from a ligand, it is advantageous to have strong field ligands coordinated to the metal to prevent energy loss via non-radiative decay pathways. Common strong field ligands are known to those skilled in this art and include CO, PPh3, and ligands where a negatively charged carbon atom bonds to the metal. N-donor ligands are also strong field ligands, although less so than those previously mentioned.
The effect of strong field ligands can be appreciated from an understanding of the mechanism by which light is emitted from a metal complex. Three reviews of luminescent metal complexes that provide an appreciation of this mechanism are referred to below.
Chem. Rev., 1987, 87,711-7434 is concerned with the luminescence properties of organometallic complexes. This review paper provides a brief summary of the excited states commonly found in organometallic complexes. The excited States that are discussed include metal-to-ligand charge-transfer (MLCT) states, which involve electronic transitions from a metal-centered orbital to a ligand-localized orbital. Thus, in a formal sense this excitation results in metal oxidation and ligand reduction. These transitions are commonly observed in organometallic complexes because of the low-valent nature of the metal center and the low-energy position of the acceptor orbitals in many ligands. Ligand to metal charge-transfer (LMCT) states also are mentioned which involve electronic transitions from a ligand-localized orbital to a metal-centered orbital.
In the section of the article that deals with photoluminescence, a sub-section is dedicated to metal carbonyl complexes, which are said to be recognized as being some of the most light-sensitive inorganic Materials. Examples include M(CO)-6 (M=V, Nb, Ta); and M(CO)6 (M=Cr, Mo, W).
Matrix isolation studies of M(CO)5L complexes, where M=Mo or W and L=pyridine, 3-bromopyridine, pyridazine, piperidine, trimethylphosphine, or trichlorophosphine, are reported also as they are said to have provided the first reports of fluorescence from substituted metal carbonyls.
Several Mo(CO)5L complexes, where L=a substituted pyridine ligand, are also mentioned and it is said that they are known to luminesce under fluid conditions. The emission has been assigned to a low-lying MLCT excited state
Other sub-sections in this review article are dedicated to dinitrogen complexes; metallocenes; metal isocyanides; alkenes; and ortho-metalated complexes.
It is said that a number of examples of ortho-metalated complexes have been shown to luminesce in room temperature solutions. For example, the emission spectrum of [Ru(bpy)2(NPP)]+ is said to exhibit the structure associated with MLCT emission. Several ortho-metalated Pt(II) complexes also are mentioned where it is said that the emission may be assigned to a MLCT excited state.
The review article summarises that low-lying MLCT excited states are often observed, because of the low-valent metal centres and vacant low-energy ligand acceptor orbitals in organometallic complexes. Further, it is reported that relationships exist between the energy ordering of the excited-state levels and the observed photophysical and photochemical properties. Still further, it is said that the great majority of examples of room temperature emission have been attributed to MLCT excited states.
Analytical Chemistry, Vol. 63, NO, 17, Sep. 1, 1991, 829A to 837A is concerned with the design and applications of highly luminescent transition metal complexes especially those with platinum metals (Ru, Os, Re, Rh and Ir).
Table I in the Analytical Chemistry paper lists representative metal complexes categorized by luminescence efficiency. The systems are limited to those containing at least one α-diimine ligand such as 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen), although many of the design rules and fundamental principles are said to apply to other classes of luminescent metal complexes.
In this paper it is explained that transition metal complexes are characterized by partially filled d orbitals and that to a considerable extent the ordering and occupancy of these orbitals determine emissive properties.
For a representative octahedral MX6 d6 metal complex, where M is the metal and X is a ligand that coordinates at one site, it is explained that the octahedral crystal field of the ligands splits the five degenerate d orbitals into a triply degenerate t level and a doubly degenerate e level. The magnitude of the splitting is given by the crystal field splitting, which is a particularly important parameter for determining the luminescence properties of the complex, whose size is determined by the crystal field strength of the ligands and the central metal ion. The luminescence properties of the complex thus can be controlled by altering the ligand, geometry, and metal ion.
There are three types of excited states mentioned in this paper: metal-centred d-d states, ligand-based π-π* states, and charge-transfer states.
Charge-transfer (CT) states involve both the organic ligand and the metal. As mentioned above, metal-to-ligand charge transfer (MLCT) involves promoting an electron from a metal orbital to a ligand orbital and ligand-to-metal charge transfer (LMCT) involves promoting an electron from a ligand to a metal orbital.
According to this paper, the most important design rule of luminescent transition metal complexes is that the emission always arises from the lowest excited state. Thus control of the luminescence properties of complexes hinges on control of the relative state energies and the nature and energy of the lowest excited state. In this regard, the paper states that any metal-centred d-d states must be well above the emitting level to prevent their thermal excitation, which would result in photochemical instability and rapid excited-state decay. Therefore, one of the more important criteria is to remove the lowest d-d state from competition with the emitting level. Thus a desirable design goal is to make the d-d state as thermally inaccessible as possible from the emitting MLCT or π-π* state. Controlling the energies of the d-d states is accomplished by varying either the ligands or the central metal ion to affect the crystal field splitting. Stronger crystal field strength ligands or metals raise d-d state energies, and crystal field strength increases in the series
Cl<py<<bpy, phen<CN<CO
where py represents pyridine.
For a metal, the crystal field splitting increases when descending a column in the periodic table. CT state energies are affected by the ease of oxidation/reduction of the ligands and metal ion. For MLCT transitions, more easily reduced ligands and more easily oxidated metals lower the MLCT states.
The π-π* state energies are largely dictated by the ligands themselves. However, the energies and intensities of the π-π* transitions can be altered by varying either the substituents, the heteroatoms in the aromatic ring, or the extent of π conjugation.
Photochemistry And Luminescence Of Cyclometallated Complexes, Advances in Photochemistry, Volume 17, 1992, page 1 to 68 describes that most of the attention in this field has been focussed on complexes of the polypyridine-type family (prototype: Ru(bpy)2+3, where bpy=2,2′ bipyridine).
The interest in the photochemical and photophysical properties of cyclometallated complexes is said to be an extension of this.
Table 2 in this publication shows absorption and emission properties of cyclometalated ruthenium, rhodium, iridium, palladium and platinum complexes and their ligands. Some of the complexes are charged and some are neutral.
Several examples exist where charged complexes have been used in LEDs or in photoluminescent studies. For example, JP 2002-203678 discloses some charged transition metal complexes. Further, in “Divalent Osmium Complexes: Synthesis, Characterisation, Strong Red Phosphorescence, and Electrophosphorescence” J. Am. Chem. Soc. 2002, 124, 14162-14172, divalent osmium complexes that feature strong red metal-to-ligand-charge-transfer phosphorescence and electrophosphorescence are disclosed. Red LEDs were fabricated by doping the Os (II) complexes into a blend of poly(N-vinylcarbazole) (PVK) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD).
Adv. Mater. 2002, 14, No. 6, March 18 “Efficient Electroluminescent Devices Based on a chelated Osmium (II) Complex” notes that among the many different classes of materials currently under investigation for the development of efficient solid state electroluminescent materials, transition metal complexes (and especially 1,2, diimine complexes of Ru) have emerged. This paper reports the fabrication and characterisation of the Os complex [Os(bpy)2L)]2+(PF6-)2, where L is cis-1,2,bis(diphenyl phosphino)ethylene. The structure for this complex is shown in the inset of FIG. 1 in this paper. This complex is reported as a luminescent material, which exhibited red-orange emission. In this paper, reference also is made to [Ru(bpy)3]2+(PF6-)2 devices.
J. Am. Chem. Soc. 2004, 126, 2763-2767 “Efficient Yellow Electroluminescence from a Single Layer of a Cyclometallated Iridium Complex” is concerned with the properties of [Ir(ppy)-2-(dtb-bpy)]+(PF6)-(ppy:2-phenylpyridine,dtp-bpy:4,4′-di-tert-butyl-2,2′-dipyridyl). According to the paper, single layer devices were fabricated and found to emit yellow light. The chemical structure of the iridium complex is shown inset in FIG. 2 in this paper.